Organopolysiloxane powder has been used as resin modifiers and paint additives and is expected to find wider applications as elastomer reinforcements and cosmetic additives. Organopolysiloxanes are solid in pure form. The organopolysiloxanes, which are insoluble in solvents, for example, have a three-dimensional molecular structure with a high degree of polymerization, generally form with organic solvents a slurry in which they are dispersed in liquid as solids. If the solvent is water, solid organopolysiloxanes are afloat on water due to their water repellency.
Organopolysiloxane powder is prepared from such organopolysiloxanes by several conventional methods including a method of separating and collecting organopolysiloxane solids by filtration or centrifugal separation, followed by drying and a hot air drying method of directly drying organopolysiloxane solids through spray dryers and similar dryers. All the conventional well-known methods for preparing organopolysiloxane powder rely on drying. More specifically, Japanese Patent Application Kokai (JP-A) No. 13813/1985 or U.S. Pat. No. 4,528,390 discloses hydrolysis of a methyltrialkoxysilane in ammoniacal aqueous solution for condensation, followed by water washing and drying. Japanese Patent Publication (JP-B) No. 27579/1991 discloses condensation of a silane with a polysiloxane in an aromatic heterocyclic compound solvent followed by collection, washing and drying.
However, where organopolysiloxanes which are solid in pure form are dissolved in a solvent to form a solution in which no solids exist, typically when the solvent is the reaction medium which is used in synthesizing the organopolysiloxanes, the organopolysiloxanes cannot be separated by conventional means such as filtration and centrifugal separation. Organopolysiloxane powder is no longer obtained.
Even in this case, organopolysiloxane powder can be yielded evaporating and removing a low-boiling liquid component or solvent from the organopolysiloxane solution. The use of hot air dryers such as spray dryers provides a continuous operation which limits the residence time of the organopolysiloxane solution within the dryer, often failing to fully remove the low-boiling liquid component.
Batchwise operation can also be employed for evaporation. Evaporation in a container free of an agitator requires a long time because as solids precipitate, solid aggregates will grow, leading to a lowering of heat transfer. Full removal of a low-boiling liquid component is not expectable because the component can be retained in such solid aggregates. At the end of operation, the organopolysiloxane which is now solid cannot be discharged from the container.
Problems will arise even when evaporation is done while agitating the organopolysiloxane solution. Conventional commonly used agitating tanks equipped with paddle or anchor impellers allow some zones to stagnate as solids precipitate, allowing solid aggregates to grow in the stagnant zones as in the previous case. This results in a lowering of heat transfer, a longer time of operation, insufficient removal of low-boiling liquid component, and difficulty to empty the tank of the solidified organopolysiloxane. Additionally, when the solution reaches a semi-solid state immediately before solidification in the course of solvent evaporation, the rotating shaft experiences increased loads. Then a powerful motor must be used for agitation and the impeller must also be thick and rigid enough to withstand the loads.
It is then proposed to use helical ribbon impellers which have been conventionally used for mixing of high viscosity liquid and powder. Since the mixing tank associated with the helical ribbon impeller is of the same shape as the conventional agitating tanks, most of the above-mentioned problems remain unsolved.
The loads on the impeller can be reduced by using conical blenders or V-shaped blenders in which the tank itself rotates. These blenders, however, are of large size, require an increased cost for installation, and are rather inadequate for heating, evaporating and purposes.
Therefore, in a situation where organopolysiloxane solids are dissolved in a low-boiling liquid component or solvent to form a solution, an object of the present invention is to provide a method for preparing organopolysiloxane powder from the organopolysiloxane solution within a relatively short time and with a relatively reduced motive power, the resulting organopolysiloxane powder being characterized by full removal of the low-boiling liquid component and easy discharge from the tank.